ASM program for Telvana

; Set up
            SEI             ; Disable interrupts
            LDA #$20        ; Set for text mode, no bitmap
            STA $D011
            LDA #$00        ; Black background
            STA $D020
            STA $D021

; Main loop
main        JSR random      ; Get a random screen position
            STA $00         ; Store low byte of address
            LDA #$04        ; High byte of screen RAM address
            STA $01
            LDX #$00        ; Counter for "ARF"

printChar   LDA message,X   ; Load character from "ARF"
            STA ($00),Y     ; Store at random position
            INX             ; Move to next character
            CPX #$03        ; Check if we printed all three chars
            BNE printChar   ; If not, loop back

            JMP main        ; Repeat indefinitely

; Simple random number generator
; Returns a random byte in A, low byte of address in Y
random      LDA $D012       ; Load current raster line
            STA $D4         ; Store it temporarily
            LDA $D4
            EOR $D4+1       ; XOR with next byte in zero-page
            ASL             ; Multiply by 2
            TAY             ; Transfer to Y to use as index
            RTS

message     .byte "ARF"     ; Message to print